Wednesday, May 30, 2007

Measuring Inductors using a frequency counter


Winding coils and finding their exact inductance is a problem for most beginners. Capacitors and resistors come marked but you have to build your own inductors using whatever comes handy.

The test oscillator shown in here is a very handy thing that can be used to easily measure the inductances and a little bit of work on a calculator.

The Circuit
None of the capacitor values are critical. If you don't have 680pf handy, try 560pf or even 100pf if those are the only ones you have. Almost NPN transistor will work. The output is attenuated down to a 50 ohms impedance through a series drop formed by the 470 ohms and a 50 ohms (actually two 100 ohms resistors tied in parallel). Althought I have shown it to work with a 9volt supply (using a PP3 battery), you can easily power it by the 12v lab suppy.

Operation
Solder the ends of the coil between the 'hot' end of the 330pf capacitor and the base of the oscillator transistor. Power up the oscillator, connect it's output to your frequency counter. Read the frequency.

Calculating the Inductance
  1. First, figure out the total capacitance in the circuit. For instance in my circuit, there are two 680pfs in series with each other, their effective capacitance will be 340pf. Then the 330pf between the ground and the inductor will also appear to be in series with the 340pf, hence the effective capacitance will be 165pf. Remember this value for all your calculations in the future.
  2. Now, divide 1000,0000 by 6.28, then divide that by the frequency read out. square it result and then divide it by the capacitance value from step 1. You have the inductance in uH (micro Henries).
There is a bit of calculation involved. But it is pretty accurate and easy way of doing it. This is essentially what all the LC meters do. Except that you have to manually calculate in our method and the LC meter does it for you.

Thoughts
This method also shows how you can make your own measurements that are as good as those possible with professional equipment if you spend just a little more effort (like using a calculator) to arrive the readings.
This is one of the many applications we will see for this interesting test circuit in combination with the frequency counter.

6 comments:

Brock R. Wood said...

Hello! This little circuit is just what I need. Thanks for posting it to your blog!

I am having trouble understanding the second paragraph of your calculation text. You state:

--- snip ---
Now, divide 1000,0000 by 6.28, then divide that by the frequency read out. square it result and then divide it by the capacitance value from step 1. You have the inductance in uH (micro Henries).
--- snip ---

By "1000,0000" do you mean "10,000,000" (ten million?) Also, when you state, "square it result," do you mean, "square the result"?

Thanks in advance!

- Brock

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